Ultrasonic waves may be used to probe a variety of materials, particularly for thickness gauging and flaw detection. The ultrasonic waves are typically generated with a piezoelectric transducer. The ultrasonic waves propagate through the material, reflecting from interfaces (in thickness gauging applications) or internal features (in flaw detection applications). The scattered ultrasonic waves propagate back to the surface of the material, causing the surface to vibrate at the ultrasound frequency. This vibration may be detected with a piezoelectric transducer similar to the one used to generate the ultrasonic waves, and then analyzed to generate data about the material.
Optical detection techniques can be used in place of the piezoelectric transducers to remotely detect the ultrasonic waves. Generally, a laser probe beam is directed onto the material. When the surface vibrates it imparts a phase shift onto the reflected beam. This phase shift is detected with a photodetector after mixing the reflected probe beam with a stable reference beam and measuring the amplitude and frequency or phase of the photodetector output intensity fluctuations. The reference beam originates from the same laser source as the reflected probe beam, and the output signal from the photodetector corresponds to the surface motion.
One problem with laser detection systems is low sensitivity. Typically, the material surface that is being probed has a diffusely reflecting or scattering quality. Consequently, the reflected beam is highly aberrated and its wavefront is mismatched with respect to the reference beam. The resulting signal produced by the photodetector is therefore weak and lacks precision.
In U.S. Pat. No. 6,075,603 to O'Meara, a contactless system for imaging an acoustic source within a workpiece is disclosed. In this system, an array of discrete optical detectors is arranged in a pattern. A probe beam is directed onto a vibrating surface in a pattern that corresponds to the detector array. The probe beam is reflected onto the detector array and a reference beam is also directed onto the detector array at an angle to the probe beam to produce fringe patterns on the detectors that correspond to the surface vibration pattern. A readout system utilizes the discrete detector outputs to produce an array output signal indicative of at least a size and two dimensional location for the acoustic source relative to the vibrating surface. This system, however, may not provide the desired accuracy, and may be sensitive to fluctuations in the length of the paths between the probe beam and the surface, and the reference beam and the surface.
U.S. Pat. No. 7,262,861 to Pepper discloses a laser ultrasonic inspection apparatus which enables remote sensing of thickness, hardness, temperature and/or internal defect detection. A laser generator impinges on a workpiece with light for generating a thermo-elastic acoustic reaction in a workpiece. A probe laser impinges on the workpiece with an annularly-shaped probe light for interaction with the acoustic signal in the workpiece resulting in a modulated return beam. A photodetector having a sensitive region is used for detecting an annularly-shaped fringe pattern generated by an interaction of a reference signal with the modulated return beam at the sensitive region.
This system, however, may not provide the desired accuracy, and may be sensitive to fluctuations in the length of the path between the probe beam and the surface, or fluctuations in the path lengths of the reference and measurement arms of the interferometer.